The present invention relates to an apparatus for processing substrates that has a pumping system for evacuating gas.
An apparatus 15 for processing a substrate 20 comprises process chambers 25a, transfer chambers 25b, and load-lock chambers 25c mounted contiguously on a platform 28 with openings for transferring substrates between the chambers, as shown in FIG. 1. In the process chamber 25a, a process gas or plasma is used to etch features, deposit layers of material on a substrate 20, or clean the chamber. The apparatus 15 is in a clean or semi-clean room 30, and a pumping system 35 used to evacuate gas and maintain the chambers at a low pressure is in an adjacent room or basement. The pumping system 35 typically comprises a high vacuum pump 40, such as a turbo molecular pump; a low vacuum pump 45, such as a rotary blower pump; and a pre-vacuum pump 50a–c, such as a dry vacuum pump. Typically, the inlet 55 of the high vacuum pump 40 is connected to the process chamber 25, and its outlet 60 to a foreline 65 that extends from the chamber to the intake 70 of the low vacuum pump 45, which in turn, is coupled to the intake of the pre-vacuum pump 50a. The pre-vacuum pump 50a exhausts to an exhaust scrubber 72. The pre-vacuum pump 50a reduces the pressure of the process chamber 25a from atmospheric pressure (760 Torr) down to a pressure of about 0.01 Torr; the low vacuum pump 45 drops the chamber pressure down to about 0.0005 Torr; and only when the chamber pressure is below 0.1 Torr is the high vacuum pump 40 operated to achieve a high vacuum below 0.1 Torr down to 10−7 Torr. Another type of high vacuum pump is the cryo pump, which is used alone or in conjunction with the turbomolecular pump. A pre-vacuum pump 50 is also used in conjunction with cryo pump (not shown) to pump down the process chambers fast. Pre-vacuum pumps 50 and low vacuum pumps 45 are most commonly used in semiconductor processing apparatus; however, some semiconductor processing apparatus also use high vacuum pumps or cryo pumps in conjunction with the pre-vacuum and low vacuum pumps. A low vacuum pump 50 is essentially a pre-vacuum pump 45 whose pumping performance is enhanced, for example, a pre-vacuum pump with an added blower can operate as a low vacuum pump 50.
The forelines 65a–c between the pumps 40, 45, 50a–c and the chambers 25a–c have a large diameter to provide a high conductance pathway that has a reduced pumping load and resistance. The low vacuum pump 45 and the pre-vacuum pumps 50 are large sized pumps that occupy volumes of from 0.5 to 1 m3, large footprint 0.5 m2, and they are noisy and vibrate excessively during operation. That is why these pumps are typically located in a separate room below or adjacent to the clean room to save clean room space and to mechanically isolate the pump vibrations from the sensitive processing equipment. The distance between the two rooms can often require a 50 to 100 feet length of foreline 65a–c. These extended lengths require that the forelines 65a–c have a large diameter and low conductance to operate the low and pre-vacuum pumps with any reasonable efficiency. Typically, the foreline 65a–c is a stainless steel pipe, which resists corrosion from the process gas, having a diameter of 50 to 100 mm (2 to 4 inches). However, the large diameter stainless steel pipe is expensive and a long length of pipe can cost as much as the pump itself. In addition, the large number of elbow joints and connections in the long foreline extending from the clean room to a separate room, have to be carefully sealed with non-corrodible gas seals to avoid leaks and releasing hazardous and toxic gases during operation, which further adds to large capital costs in semiconductor fabrication facilities. Also, the pipes are often heated to reduce the deposition of condensates on the inside surfaces of the pipes which wastes energy.
Furthermore, even with large diameter forelines 65a–c, the efficiency of the low and pre-vacuum pumps 45, 50a–c is often decreased by a factor of 2 to 4 because of the loss in pumping efficiency caused by the large length of intervening pipeline. This is especially true when the chambers are pumped down to a low pressure mTorr range, where an increase in length of the forelines 65a–c results in a large reduction in conductance. Another problem is that the large diameter and long length of the forelines 65a–c provide a large surface area that serves as a heat sink upon which condensates are deposited from the process gas flowing in the lines. These condensates are dislodged and loosened by vibrations from the pumps 45, 50a–c and back diffusion into the chambers 25a–c to contaminate and reduce the yield of the substrates 20.
Yet another problem of conventional apparatus arises because the pressure of gas in the chambers 25a–c cannot be reduced in a responsive or fast manner. Typically, the chamber pressure is measured by the pressure gauge 80 which feeds a pressure signal to a throttle valve controller 90 which opens or closes the throttle valve 75a,b to control the pressure of gas in the chamber 25a–c. However, this system is slow to respond to pressure fluctuations caused by entry of substrates 20 in the chambers 25a–c, transfer of substrates, or changes in a gas flow rate. In addition, the pressure reduction time obtained from “soft start” valves 76 are too slow. The soft gradual pressure reduction is used to prevent moisture condensation when lowering chamber pressure from atmospheric pressures to the mTorr range, by using two different size valves 76. A smaller valve opening having a low conductance is opened when pumping the chamber down from one atmosphere to about 100 to 300 Torr, and a large sized valve is opened when pumping the chamber down to lower pressures. The two-cycle process provides a soft or gradual reduction in chamber pressure in stages that minimizes moisture condensate in the chambers 25a–c. However, the time for pressure reduction during the small valve opening step of the process is often excessively long for high throughout fabrication processes.
Thus, it is desirable to have a semiconductor processing apparatus having a pumping system that does not require excessively long forelines with large diameters to operate efficiently. It is also desirable to have a small pump having reduced vibrations and noise for use in a clean room environment. It is further desirable to reduce the diameter, surface area, and length of the forelines between the chambers and the pumping system. It is also desirable to control the pressure in the chamber by means other than valves to increase response time and reduce particles. It is also desirable to more closely follow the pressure reduction versus time curve in the chamber to reduce pump down time. It is also desirable to reduce power consumption, cooling water consumption, and the release of heat within the clean room environment. It is also desirable to achieve all of the above with a small pump operating with a rotational speed of less than 10,000 revolutions per minute in order to minimize time for pressure adjustment; minimize noise, vibration, and power consumption; and maximize bearing lifetime and pump reliability.